This is forbidden by most major Kashrus authorities. Nothing is unchanged, and even those who wish to claim that it may have been Kosher, need to produce evidence that it’s currently Kosher.
Now, there is a rumour that Rav Lande from B’nei Brak permits it. I’d love to see that letter. It seems to be elusive. I think Rabbi Hasofer, who was once in Melbourne, is now the younger Rav Lande’s right hand man on Kashrus.
I note that I became a recent convert to diet tonic water (NOT for pregnant women). Schweppes used to be on the list, then were removed. It seems that much of the problem with Schweppes may be that they can’t be bothered saying what’s in their drinks and as such, we don’t know.
Enter Kirks. I bought those. Now they don’t seem to make the diet or normal tonic anymore (unless someone can direct me where I can buy).
In the meanwhile, I bit the bullet, and said to “hell” with relying on these secretive companies. We bought a SodaStream which also supports Israeli enterprise, and it has a diet tonic flavour. Provided you don’t but the fancy version with the LED lights (LED lights on Shabbos is a topic on its own) you can use it on Shabbos too.
Many of the flavours are new, and I just love the diet pink grapefruit. Reminds me of breakfast in an Israeli hotel where you fress to the extent that you can’t eat lunch.